[Bug c++/105143] New: ICE when trying to emit a [[nodiscard]] warning

["rl.alt.accnt at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105143

            Bug ID: 105143
           Summary: ICE when trying to emit a [[nodiscard]] warning
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rl.alt.accnt at gmail dot com
  Target Milestone: ---

I'm getting an internal compiler error when I try to compile the code below:
```
#include <algorithm>
#include <type_traits>
#include <string>

template <size_t sz>
struct str_lit {
    char data[sz]{};
    constexpr str_lit(const char (&_data)[sz]) { std::copy_n(_data, sz, data);
}
    template <typename str>
    constexpr bool operator==(const str& s) const {
        return sizeof data == sizeof s.data && __builtin_strcmp(data, s.data)
== 0;
    }
};

template <str_lit _name, typename _type = std::string>
struct option {
    using type                                     = _type;
    static constexpr inline decltype(_name) name = _name;
};

template <typename... opts>
struct command_line_options {
    template <str_lit s, typename... options> struct type_of;

    template <str_lit s, typename option, typename option2>
    struct type_of<s, option, option2> {
        using type = std::conditional_t<s == option::name,
            typename option::type,
            typename option2::type>;
    };

    template <str_lit s, typename option, typename... options>
    struct type_of<s, option, options...> {
        using type = std::conditional_t<sizeof...(options) == 0 || s ==
option::name,
            typename option::type,
            typename type_of<s, options...>::type>;
    };

    template <str_lit s>
    using type_of_t = typename type_of<s, opts...>::type;

    struct parsed_options {
        template <str_lit s>
        [[nodiscard]] type_of_t<s> get() { return {}; }
    };
};

using options = command_line_options<
    option<"--filename">,
    option<"--size", int64_t>>;

int main() {
    options::parsed_options opts;
    opts.get<"--filename">();
    // (void) opts.get<"--filename">(); // Doing this instead works
}
```

It seems that this happens while it is trying to emit a warning about the
unused return value of `opts.get<"--filename">()` since when I do use it or
cast it to (void) it compiles just fine. The same problem also occurs with
`[[gnu::warn_unused_result]]` instead of `[[nodiscard]]`.

Here's the backtrace:
```
'
Internal compiler error: Error reporting routines re-entered.
0xf3d164 warning(int, char const*, ...)
        ???:0
0x1190cb1 tsubst(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x1479e26 pp_format(pretty_printer*, text_info*)
        ???:0
0xf3d5cf diagnostic_report_diagnostic(diagnostic_context*, diagnostic_info*)
        ???:0
0xf953af warning_at(unsigned int, int, char const*, ...)
        ???:0
0x10fa93c convert_to_void(tree_node*, impl_conv_void, int)
        ???:0
0x10f9d03 finish_expr_stmt(tree_node*)
        ???:0
0x14e7873 c_parse_file()
        ???:0
0x14c9d9e c_common_parse_file()
        ???:0
```

I'm running gcc 11.2.0 on x86_64 Arch Linux, but I've also tested this with
several versions of GCC on compiler explorer including trunk and I ran into
much the same problem. Here's a link to that: https://godbolt.org/z/qPY4ccPfr.